Thread: The things you can learn when you try

This is a work in progress, but I thought I'd post a thread on it because while doing it I have realised that I'm using some trips, tricks and recommendations that I have picked up from others here. I'd name names but in case I get it wrong, I'll just say a general thanks (you know who you are).
The thread may be interesting to others too as it could be a future project for them as well as suggesting a few ways to help get the result they want.

Some years back now I thought I'd make a camlock adapter for my dividing head so that I could transfer chucks between lathe and dividing head without losing too much accuracy. It has served reasonably well for a while now but suffers from some inaccuracies. It recently got me into trouble with some work I was doing* for someone else so I decided to make up another and see if I could do better, aided by some of the things I've learnt over the years (*that is, I neglected to check the alignment when transferring and got caught out).

For a start, I could never work out what the thread on the dividing head was. The dividing head is a NEWS brand – a forerunner of YUASA and came with something like an 8" beaten up three jaw chuck attached. I finally got that off and adapter Mk 1 threaded on without too many dramas but at the same time was not the tightest of fits. I could never work out whether it was 57mm or 2.25", they being the two round numbers closest to what I thought it could be. Armed now with thread wires I measured it and discovered that as well as being a Whitworth form (from a profile gauge) and 8tpi it was 2.2" nominal OD.

The first task was to make up a plug gauge as close as I could to the thread of the DH. Using the SG grinding fixture that someone here had drawn my attention to, grinding a 55 degree form was straight forward. Based on recent discussions I even shaped the tool tip for better fidelity and turned the thread OD under nominal to simulate the crest form. The thread wires simplified the screw cutting task too, as I found that there was a 1:1 relationship with the thread diameter and the distance across the wires - that is, measuring the thread diameter with the wires before taking say a 2 thou pass and then measuring after would show a 2 thou difference. Having an accurately ground tool and a way of getting the diameter spot on made the job a lot easier.


The adapter is in two parts, consisting of a female thread to match the DH fastened to the cam lock nose. One of my issues with the last version was that I'd made these parts separately then expected accuracy when bolting them together. This time I cut the adapter thread (checked with the plug gauge), registered and bored out the piece that would be the camlock nose and bolted them together before finish turning.

With the level of accuracy of the equipment I have, the only way to get a spot on concentricity for adapter Mk 2 would be to turn up a dummy DH nose in the lathe and without disturbing it, mount up my adapter and turn that in situ. The dummy nose would be concentric with the lathe axis and the adapter would be concentric with the nose. Much better.
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The dummy nose started life as a a couple of pieces of bar stock. I bored the larger and then after passing the smaller diameter through, welded it up. I also put in a roll pin. The plan would be that this would sit against one of the jaws of the chuck so that it would not slip.
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The dummy nose threaded up pretty much with the same ease I had making up my plug gauge. Once that was turned I was able to mount up the nut part of the adapter and true both faces to be square to the dummy nose as well as cut the critical register diameter at one end. The nut was then turned around and another register turned to accept the camlock part. The nut was removed, the two parts bolted together and replaced on the dummy nose thread.

The rest was straightforward – the OD was turned to finish size and the start of the feature that would be the camlock taper turned. It was near the end of the day and here disaster struck. The largest diameter of the camlock taper was 2.125" and the body diameter was around 3.6" diameter. Normally I'd blaze away, get close to finish diameter, check and turn to suit – however, I over did it and when I checked I had 2.094".
Retreat to the house to think about my options. Scrapping would mean starting just about everything from scratch because as soon as I removed the dummy nose from the lathe my concentricity would be lost. I could get close with a four jaw but not spot on.
After mulling a few ideas over I decided that if I turned away more material I could heat shrink a ring on and recover. Vaguely remembering that someone had posted a rule of thumb about allowing a thou interference per inch of diameter, I decided to machine down to around 1.85" and then shrink a ring on (nominally 1.848 ID) which would take the OD back up to around 2.2".
Of course, the lathe was set up and could not be disturbed so how to get the ring made? I'd recently made up a gear for another member and had a blank lying around that had the right OD and height but needed the inside taken out. The only alternative I had was - the mill. I mounted up the rotary table and with the help of my 3D taster found the centre point. Using a pin in the chuck I then clamped a small 3 jaw chuck to it and held the blank in that. Steps of 1.5mm depth and I got there. The final cut was paper thin.
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I then deburred and turned the spigot of the camlock nose down to suit. A little gentle heat and it slid on. In hindsight I could have had a bigger interference but it held. Leaving some time to cool I trimmed everything up and then set up the taper. The taper is 3" per ft (inclusive) so I worked out that if I marked out a 2" length on the side of the turned adaptor, 250 thou movement of my indicator would be right. The plan worked well. The photo may not show it well but the black texta mark (there is another at the other end) has a faint scratch mark in it so I had 2" marked to the thickness of the scratch.
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Turning the taper was an anticlimax. I used an old (but little used) backing plate as a gauge with the candle black trick. Messy, but very gratifying to see that the angle set and the backing plate agreed closely. Once I had established that the angle was correct I then used marker pens on the taper to gauge how much material I'd taken off.
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The turning part is done. Now that I have a concentric part I'll mount it on the Dividing head to get the holes in the right position and I should be set. I checked the threaded nose of the DH and there is around 0.01mm TIR on the register. If I can keep things in that neighbourhood, I will be chuffed


Michael